MEMS Gyroscope for Detecting Rotational Motions about an X-, Y-, and/or Z-Axis

ABSTRACT

The invention relates to a MEMS gyroscope for detecting rotational motions about an x-, y-, and/or z-axis, in particular a 3-D sensor, containing a substrate, several, at least two, preferably four, drive masses ( 2 ) that are movable radially with respect to a center and drive elements ( 7 ) for the oscillating vibration of the drive masses ( 2 ) in order to generate Coriolis forces on the drive masses ( 2 ) in the event of rotation of the substrate about the x-, y-, and/or z-axis. The oscillating drive masses ( 2 ) are connected to at least one further non-oscillating sensor mass ( 3 ) that however can be rotated about the x-, y-, and/or z-axis together with the oscillating drive masses ( 2 ) on the substrate. Sensor elements ( 9, 10 ) are used to detect deflections of the sensor mass ( 3 ) and/or drive masses ( 2 ) in relation to the substrate due to the generated Coriolis forces. At least two, preferably four anchors ( 5 ) are used to rotatably fasten the sensor mass ( 3 ) to the substrate by means of springs ( 4 ).

The present invention relates to a Micro-Electro-Mechanical System orMEMS gyroscope for detecting rotational movements about an x, y and/or zaxis, particularly as a three-dimensional sensor, with a substrate andwith several actuator driving masses which vibrate in an oscillatorymanner in order to produce Coriolis forces on the driving masses duringrotation of the substrate about the x, y and/or z axis.

A three-dimensional micro-electro-mechanical MEMS gyroscope is knownfrom TW 286201 BB. This has masses that are arranged on a centralarmature and that are caused to move in an oscillating rotary motion.The masses are arranged on a substrate and are tilted about the y or xaxis when a torque is applied about the x or y axis due to a resultingCoriolis force. This is made possible by suitable suspension of thesedriving masses on the substrate. When a torque occurs that acts aboutthe z axis, partial masses can be deflected with a translation movementas a result of another suitable suspension of these partial masses onthe rotationally mounted masses. Both the tilting movements and thetranslational motion can be detected by sensors and can be used as ameasure of the corresponding rotation about the x, y or z axis becauseof their proportionality to the rotational movement of the substrate.The respective displacements, however, are very difficult to determine.

In order to create a three-dimensional gyroscope as a three-dimensionalsensor with which rotations can be detected in all three axes, D. Woodet al have proposed in 1996 in the article “A monocyclic silicongyroscope capital of sensing about three axes simultaneously” agyroscope which has oscillating masses arranged annularly around acentral anchoring point These masses are capable of carrying out bothtilting and rotational movements as a result of Coriolis forces thatoccur. The disadvantage is that the manufacture of such a sensor, aswell as the drive for the moving masses, is difficult. The movements ofthe individual components of the sensor mutually influence each other,so that measurements of the movement in the x, y or z direction of thegyroscope do not provide sufficient accuracy.

The object of the present invention is to create a MEMS gyroscope fordetecting rotational movements about an x, y and/or z axis, particularlyas a three-dimensional sensor, with a high degree of detection accuracy.

The object is solved with a MEMS gyroscope with the features of Claim 1.

According to the invention, the MEMS gyroscope for detecting rotationalmovements about an x, y and/or z axis comprises a substrate and several,at least two or preferably four, driving masses that are movableradially relative to a central point. Drive elements set the drivingmasses into an oscillating vibration as the primary oscillation in orderto generate Coriolis forces acting on the driving masses when a rotationof the substrate about the x, y and/or z axis occurs. The oscillatingdriving masses are connected to at least one other non-oscillatingsensor mass, which, together with the oscillating driving masses, canrotate on the substrate about the x, y and/or z axis. Sensor elementsare provided on the MEMS gyroscope in order to detect displacements ofthe sensor mass and/or the driving masses in relation to the substrateas a result of the generated Coriolis forces as a secondary vibration.The sensor mass is equipped with at least two, preferably four anchoringdevices for rotary attachment to the substrate by means of springs.

According to the invention, the oscillating driving masses are attachedto the sensor mass and can oscillate independently of the sensor mass.The sensor mass itself is finally attached to the substrate via at leasttwo anchoring devices. The attachment to the anchoring devices iseffected via springs, which allow movements of the sensor mass togetherwith the driving masses relative to the substrate. These movements takeplace as a rotary motion about the z-axis emanating from the plane ofprojection and as a tilting movement about the x and y axis lying in theplane of projection. As a result of this, independent rotationalmovements of the substrate or the gyroscope about an x-axis, a y-axisand/or a z-axis are to be determined by the sensor elements, which arearranged at the appropriate place. In particular in the configurationwith four anchoring devices, by means of which the sensor mass isarranged on the substrate, a balanced mounting of the masses on thesubstrate is effected. The displacement of the masses resulting from theCoriolis forces occurring during a rotary motion of the substrate takesplace uniformly in all directions, so that the deflection movements inall directions are of a similar type. This is especially beneficial ifthe four anchoring points are arranged on the x axis and the y axis,whereby two of the anchoring points are arranged on each one of theseaxes. This causes the tilting movement about the x axis to be of asimilar type to the tilting movement about the y axis.

The springs, with which the sensor masses are attached to the anchoringdevice, enable the described tilting movement about the x axis and yaxis and a rotation about the z axis to take place. However, they arestiff in their interaction with respect to translational movements inthe x or y direction. In this way, a stable system is created which isstiff in relation to translational movements but compliant in relationto the rotational movements of the sensor mass.

In a preferred embodiment of the invention, the sensor mass encloses thedriving masses in the form of a framework. This allows the drivingmasses to be well accommodated in the mass sensor and ensurestrouble-free operation of the sensor. The framework also ensures thatthe driving masses are smoothly operated and supported, so that theresulting Coriolis forces and hence the corresponding tilting movementscan be smoothly passed from the driving masses to the sensor mass.

If the driving masses are arranged point symmetrically in pairs relativeto the centre of the sensor, then drive vibrations can be easilybalanced out so that the sensor has an essentially static sensor masswhen in the resting position. The sensor mass is thus not adverselyaffected by the moving driving masses or even set into vibration unlessCoriolis forces occur.

The sensor elements for detecting the displacement of the masses aboutthe x axis or y axis are preferably arranged below the sensor massand/or below the driving masses as horizontal plate capacitances or asvertical capacitances in the sensor mass. For each of the arrangementsreferred to, the displacement of the masses about the x axis or y axisin their secondary vibrations can be detected as a change in capacitanceor electrical voltage. The corresponding amplitude acts as a measure ofthe rotation rate acting on the rotation rate sensor.

Sensor elements are preferably arranged within or outside the sensormass as vertical capacitances or as comb electrodes in order to detectthe displacement of the masses about the z axis. The rate of rotationabout the z axis can be derived from a change in the electricalamplitude.

The sensor mass is preferably fixed on the substrate using flexuralsprings. The flexible springs ensure that the sensor mass is arranged onthe substrate by means of the corresponding anchoring device in a stablemanner. The individual flexural springs are intended to work together insuch a way that ideally the sensor mass and the driving masses have nocontact with the substrate if they are deflected owing to the vibrationscaused by the Coriolis force.

The flexural springs are preferably designed to allow a rotation of thesensor mass about the x, y and z axes. This enables rotary movements ofthe substrate as a secondary movement resulting from the Coriolis forcesoccurring in an oscillating rotation of the sensor mass about the x, yand/or z axis.

The flexural springs are preferably configured to prevent displacementof the sensor mass in the x, y and/or z direction. The flexural springsare rigidly constructed in their interaction with regard to atranslational displacement of the sensor mass on the substrate, in orderto prevent a displacement of the sensor mass in a linear direction onthe substrate.

In particular, to enable good, sensitive displacement of the sensormass, provision is made for the anchoring devices to be arranged in theregion of the centre of the sensor. This allows the flexural springs tobe configured with a suitable length, to have a low spring constant inthe desired direction and therefore to be relatively soft. Bending isthus easily possible in the event of a suitable force acting on them.The sensor mass suspended from the springs can therefore be easily andrepeatably tilted, even by small Coriolis forces.

The anchoring devices are preferably arranged between the drivingmasses. This yields a uniform distribution of anchorage points andmoving masses. The displacement of the sensor mass can thus take placeuniformly and systematically in all directions.

In a particularly preferred embodiment of the invention, the drivingmasses are attached to the sensor mass by flexural springs that exhibitelasticity in the drive direction. The driving masses can thus be drivenin an oscillatory manner in the drive direction, without exerting arelevant influence on the sensor mass. The drive movement of the drivingmasses thus causes no direct movement of the sensor mass, although themasses are connected to each other by the flexural springs. The flexuralsprings are, however, relatively rigidly connected to the sensor mass indirections that deviate from the drive direction, so that the Coriolisforces acting on the driving masses can contribute to a tilting orrotary motion of the sensor mass together with the driving masses.

In a most particularly preferred embodiment of the invention the drivingmasses are linked to each other by synchronising springs. Thisadvantageously enables the movements of the driving masses to occur insynchrony with each other, so that no reaction forces resulting fromunequal movements of the driving masses act on the sensor mass. Thiswould lead to displacements of the sensor mass that are not caused bythe Coriolis forces. Undesirable measurement errors would result fromthis. However, the synchronising springs connect the individual drivingmasses together, so that the drive movements of the individual drivingmasses are equal and balance each other with respect to the forcesoccurring.

It is particularly advantageous if the synchronising springs arearranged very close to the centre. In particular, if they are closer tothe centre than the anchoring device of the sensor mass, this willensure that the synchronising springs and the flexural springs of thesensor mass do not obstruct each other. Moreover, the flexural springs,in the same way as the synchronising springs, are long enough that theyare sufficiently elastic in the relevant direction and both the mobilityof the sensor mass as well as the synchronization and mobility of thedriving masses are guaranteed.

If the driving directions of the driving masses are inclined relative toeach other, with four driving masses preferably at a 90° angle or withthree driving masses preferably at a 120° angle, this ensures that thedriving masses can be operated uniformly without any forces acting onthe sensor mass that are not caused by Coriolis forces.

A particularly stable system results from the fact that, in apreferential embodiment of the invention, the driving directions of thedrive elements are arranged at a 45° angle to the x/y axes. This systemreacts very sensitively to Coriolis forces and is capable of indicatingCoriolis forces even at low rotation rates about the appropriate axis.

The drive elements of the driving masses are preferably electrodes, inparticular fork or comb electrodes. Some of the electrodes are attachedto the substrate and other electrodes are arranged on the driveelements. By the application of an alternating voltage, the electrodesare attracted and repelled, producing an oscillating movement of thedriving masses.

Other advantages of the invention are described in subsequent embodimentexamples. These show:

FIG. 1 a MEMS gyroscope according to the invention in plan view,

FIG. 2 another MEMS gyroscope in plan view and

FIG. 3 another MEMS gyroscope in plan view.

FIG. 1 shows the plan view of a three-dimensional MEMS gyroscope 1. Inparticular, it shows the moving parts of the gyroscope 1, namely fourdriving masses 2 and a sensor mass 3. The sensor mass 3 encloses thefour driving masses 2 in the manner of a frame work. The driving masses2 are located within the sensor mass 3.

The sensor mass 3 is arranged on a substrate (not shown) via flexuralsprings 4 and anchoring devices 5. The flexural springs 4 are flexurallycompliant in a direction transverse to their longitudinal extension. Intheir longitudinal extension, however, they are stiff. This causes thesensor mass 3 to be rotatable about an x axis and a y axis lying withinthe plane of projection, as well as about a z axis emanating from theplane of projection. The oscillating rotary movements are represented bycorresponding arrows.

The driving masses 2 are arranged so as to be framed by the sensor mass3. The driving masses 2 are attached to the sensor mass 3 by flexuralsprings 6. Each of the driving masses 2 has four of these flexuralsprings 6. The driving masses 2 are driven in an oscillating motion inthe direction of the double arrow by drive elements 7. The driveelements 7 consist, for example, of comb electrodes, some of which areattached to the substrate and others to the driving mass 2, and theytherefore set the driving mass 2 into an oscillating vibration by meansof an applied alternating voltage.

The flexural springs 6 are designed to be flexurally elastic in thedriving direction of the driving masses 2, but to be stiff in all otherdirections. This causes the driving mass 2 to be largely free tooscillate in the drive direction, while in the other directions thedriving mass 2 is coupled to the movements of the sensor mass 3. Thesensor mass 3 together with the driving masses 2 is thus rotated as asecondary movement in a corresponding direction by a Coriolis forcewhich arises during a rotational movement of the substrate about one orseveral of the x, y and/or z axes.

The four driving masses 2 are arranged in the sensor mass 3 such thatthey vibrate in opposition to each other in pairs and are arranged pointsymmetrically relative to the z-axis. In this way, forces and torquesthat could result from the movement of the driving masses 2 cancel eachother out, and the sensor mass 3 is not set in motion owing to the drivemotion of the driving masses 2 alone.

In order to achieve this balance and thus to ensure that the sensor mass3 is stationary, the driving masses 2 are joined together bysynchronising springs 8. The synchronising springs 8 are arranged on thez-axis between the anchoring device 5 and the centre of the gyroscope 1.This ensures that they do not interfere with the movement of theflexural springs 4 and the anchoring device 5. The synchronising springs8 are formed in a U-shape. A periodic movement of the two driving masses2, which are linked together by the synchronising springs 8, towards andaway from each other produces a varying distance between the two drivingmasses 2. The synchronising springs 8 can be spread accordingly duringthis process, due to their shaping. The synchronising springs 8 exertforces on the driving masses 2 with the result that differences in speedare compensated and hence the drive movements of the four driving masses2 take place synchronously.

Plate capacitances 9 are arranged in the region of the x and y axesbelow the sensor mass 3. An electrical signal is generated by theseplate capacitances 9 as soon as the sensor mass 3 rotates about the x ory axis. This signal is proportional to the Coriolis force that arises asa result of a rotation of the substrate about the x or y axis. In orderto detect the rotational movement of the sensor mass 3 about the z axis,comb electrodes, for example, are provided, especially at the peripheryor outer area of the sensor mass 3, which detect a rotary movement ofthe sensor mass 3 about the z axis in the form of an electrical signaland allow conclusions to be drawn regarding a corresponding rotation ofthe substrate.

By attachment of the sensor mass 3 on the anchoring device 5 of thesubstrate by means of the flexural springs 4, and by attachment of thedriving masses 2 by means of the flexural springs 6 to the sensor mass3, a system is produced in which the primary movement of the drivingmass 2 is maximally decoupled from the secondary movement, which arisesas a result of the driving masses 2 and the coupling to the sensor mass3. The rotational movement of the driving masses 2 and sensor mass 3 asa response to a rotation of the gyroscope 1 or of the substrate, towhich the sensor mass 3 and the driving mass 2 are attached, can bedetected without its being disturbed by the drive movement of thedriving masses 2.

After the secondary movement has taken place, coupled via the drivingmass 2 and the sensor mass 3, there is also a possible alternative tothis example, whereby the sensor elements, in this case the platecapacitances 9, are arranged not only below the sensor mass 3, but alsobelow the driving mass 2. This also clearly applies to the sensorelements that detect the rotation around the z-axis. Again, the motionis jointly carried out by the driving masses 2 and the sensor mass 3, sothat this movement can also occur in the region of the driving massesand/or the sensor mass 3.

FIG. 2 illustrates an alternative example to the MEMS gyroscope shown inFIG. 1. Similar components are denoted using the same reference labelsas are used in FIG. 1. Just as in FIG. 1, the MEMS gyroscope 1 has fourdriving masses 2, which are each connected to the sensor mass 3 by meansof four flexural springs 6. The sensor mass 3 encloses the four drivingmasses 2 in the manner of a frame. The driving masses 2 move at an angleof 45° to the x and y axes in a direction that extends through thecentre of the gyroscope 1 in the region of the z axis, and towards andaway from the centre. Opposite driving masses 2 move in an oscillatingmotion in opposite directions so as to prevent vibrations from occurringon the sensor mass 3.

The sensor mass 3 is anchored using one flexural spring 4 each to atotal of four anchoring devices 5 such as to be able to rotate and tiltabout the x, y and z axes. The synchronising springs 8, which extendbetween the anchoring devices 5 and the centre of the gyroscope 1,ensure that the driving masses 2 oscillate synchronously with respect toeach other. The flexural springs 6, with which the driving masses 2 areconnected to the sensor mass 3, allow oscillating movements of thedriving masses 2 in the drive direction, but are rigid in all otherdirections, such that Coriolis forces that occur can be transmitted fromthe driving masses 2 to the sensor mass 3.

A further embodiment of a three-dimensional MEMS gyroscope 1 accordingto the invention is illustrated In FIG. 3. The connection of the drivingmasses 2 to the sensor mass 3 and the connection of the sensor mass 3via the flexural springs 4 and the anchoring devices 5 on the substrateare made in a similar way as in the exemplary embodiments in FIGS. 1 and2. The synchronising springs 8 are formed in the shape of an arrow inthis embodiment, but are also oriented towards the centre of thegyroscope 1. The detection of the rotary motion of the sensor mass 3 anddriving masses 2 about the x, y or z axis is carried out in thisembodiment of the invention by means of sensor elements 10, which arearranged within the sensor mass 3. These sensor elements 10 are, forexample, vertical capacitances, which produce variable electricalsignals in the event of a rotary movement of the sensor mass 3 about thez axis. With a suitable configuration, a rotation about the x or y axisof the sensor mass 3 can be detected with these sensor elements 10 orwith similar sensor elements, or even with plate capacitances, asillustrated in the version in FIG. 1.

The invention is not limited to the illustrated examples. In particular,the number of driving masses can thus be different from the number shownhere. In addition, the manner of detecting the rotational movement ofthe sensor mass 3 may be different than that illustrated here. Thedesign of the driving masses 2 and the sensor mass 3 is furthermore notnecessarily angular, but can be rounded or circular in anotherembodiment of the invention. In addition to the foregoing, the inventionrelates to all embodiments that are formed according to the currentclaims.

1. MEMS gyroscope for detecting rotary movements about an x, y and/or zaxis, particularly as a three-dimensional sensor, with a substrate, withmultiple, at least two, preferably four driving masses (2) movableradially relative to a central point, with drive elements (7) for theoscillating vibration of the driving masses (2) in order to generateCoriolis forces on the driving masses (2) in the event of rotation ofthe substrate about the x, y and/or z axis and wherein the oscillatingdriving masses (2) are connected to at least one additional sensor mass(3), which is non-oscillating but which can be rotated on the substrateabout the x, y and/or z axis together with the oscillating drivingmasses (2) and with sensor elements (9, 10), in order to detectdisplacements of the sensor mass (3) and/or the driving masses (2) inrelation to the substrate as a result of the generated Coriolis forces,and with at least two, preferably four anchoring devices (5) for therotary attachment of the sensor mass (3) to the substrate by means ofsprings (4). 2-15. (canceled)